1. Technical Field
Example embodiments of the present invention relate to methods of performing communication in a wireless local area network (LAN), and more particularly, to methods of performing cooperative communication in a wireless LAN that is capable of improving the throughput of the wireless LAN.
2. Related Art
Wireless LANs such as IEEE 802.11a/b/g-based wireless LANs can support multiple transmission rates depending on channel conditions. For example, the IEEE 802.11b-based wireless LAN can support four transmission rates: 1, 2, 5.5, and 11 Mbps.
One problem of wireless LANs supporting multiple transmission rates is low average throughput of the overall network in view of a maximum transmission rate that can be supported by the networks. In particular, the average throughput of the overall network is considerably reduced according to a number of low-rate terminals within the transmission coverage of an access point (AP), in spite of presence of high-rate terminals. The degradation of the average throughput of the network, which may be a common problem of IEEE 802.11-based wireless LAN series supporting multiple transmission rates, results from a carrier sense multiple access with collision avoidance (CSMA/CA) scheme that was adopted as a basic medium access scheme by the IEEE 802.11 series.
CSMA/CA provides fair transmission opportunity to all terminals through a binary exponential backoff algorithm. CSMA/CA is an excellent medium access scheme because CSMA/CA provides fair transmission opportunity to all terminals, but has a characteristic that the throughput of the overall network is affected by low-rate terminals because the low-rate terminals have a longer channel occupying time than high-rate terminals.
FIG. 1 illustrates an operational process of legacy IEEE 802.11 MAC protocol. Referring to FIG. 1, a transmitter (station 1 or a first source) obtains transmission opportunity after backoff contention subsequent to a distributed coordination function InterFrame space (DIFS) through a binary exponential backoff algorithm-based CSMA/CA, and then transmits a request to send (RTS) frame to a receiver (a destination or an AP) to reserve a medium.
Upon successful receipt of the RTS frame, the receiver responds with a clear to send (CTS) frame after a short InterFrame space (SIFS) to request the transmitter to transmit data.
In response to the CTS frame, the transmitter transmits the data to the receiver through the medium, and upon successful receipt of the data, the receiver transmits an ACK frame to the transmitter.
Then, another transmitter (a second source) transmits data to the receiver (a destination or an AP) through the same process as described above.
As shown in FIG. 1, as the legacy IEEE 802.11-based MAC protocol has the characteristic of CSMA/CA that fair transmission opportunity is provided to all terminals, all the terminals have the same channel access probability. A low-rate terminal successfully accessing the channel has a longer channel occupying time than a high-rate terminal. As terminals accessing the channel increase, the overall throughput of the network decreases.
A method of setting a different size of an initial contention window according to a transmission rate (different initial contention window size adaptation; DICWA), and a method of limiting a maximum data size that can be transmitted in a unit time frame according to a transmission rate (different MAC service data unit size adaptation; DMA) have been proposed to solve the problem described above. Recently, effective combinations of the two methods have been studied.
Also, a method of increasing overall throughput of a network through cooperative communication as a next-generation communication scheme has recently been studied. Among methods of improving average throughput of a network through cooperative communication in a wireless LAN, a method of embodying cooperative communication through cooperation between terminals has been proposed. In this method, a high-rate terminal relays data from a low-rate terminal to an AP.
However, overhead may additionally occur when performing communication through cooperation between terminals in a wireless LAN. Since the overhead may degrade overall throughput, it is necessary to provide a method of minimizing the overhead. Also, since terminals in a wireless LAN are privately owned, terminals participating in relaying to increase a transmission rate for other users through cooperation between the terminals must be properly rewarded.